The invention relates to MOSFET sampling switch circuits, and particularly to a bootstrapped MOSFET sampling switch circuit that operates on a single power supply voltage, has an input signal that ranges above and below a ground voltage, and nevertheless produces a constant channel resistance in the MOSFET over a range of input voltages, and hence results in low total harmonic distortion of a sampled input signal.
FIG. 2 shows the connections during the "tracking" or "sampling" mode of a conventional analog sampling switch that includes an N channel MOSFET 6 having its gate electrode connected to +V, its source connected by conductor 17 to receive an analog input voltage V.sub.IN, and its drain connected by sampling capacitor 13 to an output conductor 15. Conductor 15 is connected to a CDAC switched capacitor array represented by numeral 16 and to a comparator having its output coupled to a successive approximation register (not shown). The bulk or body electrode of MOSFET 6 is connected to -V volts.
A problem with the circuit of FIG. 2 is that the gate-to-source voltage (which partially determines the channel resistance R.sub.DS of MOSFET 6) is quite low, typically only a few volts. The gate-to-source voltage of MOSFET 6 varies considerably as V.sub.IN varies. This gate-to-source voltage variation causes a varying channel resistance of sampling MOSFET 6. Furthermore, the threshold voltage V.sub.TH of MOSFET 6, which also influences the channel resistance, is a strong function of the voltage between its source electrode and its body electrode. The source-to-bulk voltage varies considerably as V.sub.IN varies, producing further changes in the channel resistance of sampling MOSFET 6 as a function of the input voltage V.sub.IN. Consequently, a considerable amount of harmonic distortion is produced in the sampled version of V.sub.IN that is stored on sampling capacitor 13 by charging it through the non-linear channel resistance R.sub.DS of sampling MOSFET 6.
If conductor 15 is connected to the capacitor array of a CDAC (Capacitor Digital-to-Analog Converter), such harmonic distortion produces errors in the digital output word produced by the analog-to-digital converter to represent the time-varying analog input voltage V.sub.IN.
It would be desirable to provide an analog sampling switch circuit that produces a constant channel resistance in a switching MOSFET even though the circuit operates from a single 5 volt power supply, and even though the analog input signal might be a sinusoidal waveform having bipolar values between +10 volts and -10 volts. Although the sampling MOSFET 6 can be made more linear by bootstrapping its gate voltage in somewhat the same manner described in commonly assigned co-pending allowed patent application Ser. No. 602,705, filed Oct. 24, 1990, entitled "DYNAMIC INPUT SAMPLING SWITCH FOR CDACS", that circuit requires a continuous clock signal to keep the bootstrap circuit operating, and will not work if the sampling device is turned on intermittently. That circuit requires a +5 volt power supply and a -5 volt power supply.